While colonoscopy—the gold standard for screening—is very effective at detecting cancer, regular exams are recommended only starting at age 50, and then every 10 years. Although 90 percent of cases appear in people age 50 and older, you should be concerned about worrisome symptoms at any age, says Patricia Raymond, MD, a fellow at the American College of Gastroenterology. “People do get it in their 20s, 30s and 40s,” she says. And while rectal bleeding is the most obvious red-flag symptom, Raymond says these three others could indicate that something is wrong.

Weakness, lightheadedness, or dizziness. It’s easy to brush off these feelings as “nothing” since they’re so common, but Raymond said if they persist for an abnormally long time or get worse, you should get checked out. The reason many people with colon cancer don’t experience obvious symptoms right away is because they could be bleeding very slowly. The level of hemoglobin in their blood goes down, which means fewer red blood cells to carry oxygen. “Quite often people can lose blood for a long period of time before they’re diagnosed,” says Raymond.

Shortness of breath. This is another side effect of a slow bleed and is a sign you could be anemic. If you aren’t bleeding aggressively or vomiting blood, your body will just put more plasma in the blood but won’t make more iron or red blood cells. That “compensation” prevents you from losing blood in large volumes but takes away the blood’s ability to carry oxygen, which is why you might notice shortness of breath.

Skinny stool. Pay attention to what’s in the toilet, even if you don’t see blood, says Raymond. A change in diameter to a narrow or skinny shape could point to a restriction in the colon caused by polyps.

Other symptoms include blood in or surrounding the stool, unexplained weight loss, and stomach pain or cramps that don’t go away.

And if you have any of the following risk factors, it’s even more important to watch out for colon cancer symptoms: personal or family history of colorectal cancer, inflammatory bowel disease, eat a lot of red or processed meat, have a low fiber high fat diet.